1. Field of the Invention
The invention relates to adhesive compositions and adhesive tapes suitable for use as insulating tapes.
2. Description of the Art
Various types of adhesives are known in the art for use as insulating tapes.
U.S. Pat. No. 4,732,936 discloses copolymers of alpha methylstyrene and paramethyl styrene. The copolymers are disclosed to be especially useful as reinforcing or modifying resins for block copolymers having aromatic and aliphatic blocks contained in pressure-sensitive adhesives. Examples of block copolymers include styrene-isoprene-styrene, styrene-butadiene-styrene and styrene-ethyl-butylene-styrene.
U.S. Pat. No. 4,717,749 discloses a pressure-sensitive adhesive composition comprising an ABA block polymer wherein A represents a copolymer of styrene and alpha-methylstyrene and B represents blocks of diene polymer. The diene polymer can be butadiene, isoprene or a mixture thereof. A tackifying resin is also present in the composition.
U.S. Pat. No. 4,102,835 discloses a pressure-sensitive adhesive having improved physical strength provided by adding 5 to 30 parts of an ABA block copolymer to the normally tacky rubber polymers. Isoprene, polybutadiene and the like are disclosed as base polymers; preferred block copolymers include styrene-butadiene copolymers, styrene isoprene polymers and the like.
U.S. Pat. No. 4,181,635 discloses a pressure-sensitive adhesive composition having excellent tack and improved thermal resistance comprising a low molecular weight isoprene and an elastomer. Useful elastomers include polybutadiene, polyisoprene, styrene-butadiene rubbers, styrene-isoprene copolymers, and the like.
U.S. Pat. No. 4,060,503 discloses an adhesive composition comprising an admixture of at least one natural rubber and synthetic rubber with a tackifying resin containing 2-12% of an alpha-methylstyrene modified piperylene/2-methyl-2-butene resin, which is disclosed to enhance the tack.
Japanese Patent Kokoku 59-28236 (translated) discloses isoprene pressure-sensitive adhesives having a blend containing 20-65% cis-1,4-polyisoprene rubber and correspondingly, 80%-35% styrene-isoprene block copolymer, and from 10% up to 250% tackifier. The composition is disclosed to have a better balance of cohesion and adhesion properties; however, if the lower limit of the tackifier is indeed 10%, such compositions would not be pressure-sensitive adhesives.
U.S. Pat. No. 4,136,071 discloses a pressure-sensitive adhesive comprising 50-90 parts of a linear or radial styrene-isoprene-styrene copolymer, 10-50 parts of a simple styrene-isoprene copolymer. It is further disclosed that the adhesive may include small amounts of other elastomers, but these should not exceed 25%.
U.S. Pat. No. 3,660,323 discloses a pressure-sensitive adhesive which includes a blend of a polyisoprene, a polystyrene-polyisoprene-polystyrene block copolymer and natural rubber. The natural rubber is a critical ingredient said to improve the adhesion of the mass to the backing without detracting from tack, holding power and absence of allergenic characteristics.
U.S. Pat. No. 3,932,328 discloses a pressure-sensitive adhesive which comprises a C5 solid tackifier, a liquid tackifier, and 100 parts of a styrene-isoprene-styrene elastomer. However, these adhesives show low adhesion to plasticized vinyl backings, and have a raspy unwind which is considered undesirable for an insulating tape.
It is also known that flexible vinyl substrates containing plasticizers cause problems for adhesives as they leach from such substrates, contaminating the adhesives. This is especially true for low molecular weight monomeric plasticizers, which are widely used because they are inexpensive. Adhesives typically show a significant decrease in adhesion over time as the plasticizer leaches into the adhesive and softens it.
Various solutions have been suggested for this problem, including interposing an impenetrable barrier layer, as disclosed in U.S. Pat. No. 4,045,600. It has also been suggested that incorporating plasticizer directly into the adhesive will reduce the gradient of plasticizer between contacting layers, and prevent migration. See, e.g., U.S. Pat. Nos. 4,272,573 and 4,946,742.
The present inventors have now discovered that "smooth unwind" adhesives useful as adhesive compositions, especially and as adhesives for insulating tapes, may be made by combining polyisoprene homopolymers, certain elastomeric block copolymers, solid tackifiers, and a reinforcing end block resin.
It has further been discovered that such adhesives when placed on a substrate, form a tape which shows improved resistance to softening caused by plasticizers over conventional insulating tapes, while continuing to exhibit good adhesive properties.